1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an electrophotographic image forming apparatus, such as an electrophotographic copying apparatus, an electrophotographic printer, comprising means for automatically adjusting an image reproduction density so as to always obtain a stable gradation reproducibility.
2. Description of the Related Art
Conventionally, there have been put into practical use various kinds of electrophotographic digital image forming apparatuses such as a laser printer, each using an electrophotographic process of reversal development type for driving a laser diode based on digital image data of an image of an original and reproducing the image of the original on a sheet of printing paper. Further, there have been proposed various kinds of digital image forming methods for faithfully reproducing a half-tone image such as a photograph.
As the digital image forming methods of these type, there have been known to those skilled in the art, an area gradation method using a dither matrix, and multi-value laser exposure methods such as a pulse width modulation method for representing a gradation of one dot image to be printed by changing a pulse width or an emitting time of a beam of laser light so as to change a light amount thereof defined as a product of the emitting time and an emitting intensity, and an intensity modulation method for representing a gradation of one dot image to be printed by changing an emitting intensity of a beam of laser light so as to change a light amount thereof (See Japanese Patent Laid-open Publication Nos. 62-91077, 62-39972, 62-188562 and 61-22597). Further, there has been publicly known a multi-value dither method which is a combination of the dither method and the above-mentioned pulse width modulation method or the above-mentioned intensity modulation method.
In the gradation method of this type for representing a gradation, it is considered possible in principle to reproduce an image density having a gradation strictly corresponding to a gradation of image data to be reproduced, however, an actually reproduced image density (referred to as an image reproduction density hereinafter) is not correctly proportional to an original density to be reproduced because of a complicated combination of characteristics of a photoconductor and toners and circumstances etc. In other words, a relationship between the image reproduction density and the original density is shifted from a linear characteristic curve to be originally obtained. Such shifted characteristic as described above is generally called a .gamma. characteristic, which mainly causes deterioration of faithfulness of reproduced images of originals, particularly a half-tone original.
Therefore, in order to improve faithfulness of a reproduced image, conventionally, there has been performed a so-called .gamma. correction process for converting data of a read original density into data using a predetermined .gamma. correction table and forming a digital image of dot images based on the converted data of the original density so that the relationship between the original density and the image reproduction density becomes linear. Thus, normally, the image of the original can be faithfully reproduced depending on the original density by performing the above-mentioned the .gamma. correction process.
On the other hand, as one of phenomena due to another cause for influencing the image reproduction density, there is known such a phenomenon that an adhering toner amount onto the photoconductor changes upon a developing process using the toner when characteristics of the photoconductor and the toner change due to change in external circumstances such as the temperature, the humidity, etc. Generally speaking, the adhering toner amount increases under circumstances of a high temperature and a high humidity so that the original image having a higher image reproduction density is reproduced with a .gamma. characteristic having a relatively large gradient in a relatively high original density. On the other hand, the adhering toner amount decreases under circumstances of a low temperature and a low humidity so that the original image having a lower image reproduction density is reproduced with a .gamma. characteristic having a relatively small gradient in relatively low and middle original densities.
Thus, there is such a problem that the reproduced image density changes due to change in the circumstances. In order to solve the above-mentioned problem so as to obtain a stable proper image reproduction density, there has been performed an image density control process for controlling the maximum image reproduction density to be constant, generally, in a conventional electrophotographic copying apparatus, a conventional electrophotographic printer, or the like.
One of the above-mentioned image density control processes which have been put into practical use will be described below with reference to FIG. 5 for illustrating an image forming part comprising a photoconductive drum 41 and a developing roller 45r.
Referring to FIG. 5, a corona charger 43 having a discharging electric potential V.sub.C is provided so as to confront a photoconductive drum 41. A negative grid voltage V.sub.G is applied to a grid of the corona charger 43 by a grid voltage V.sub.G generator 243. Since it is considered that a surface electric potential Vo on the surface of the photoconductive drum 41 immediately after electrically charging the photoconductive drum 41 by the corona charger 43 and prior to an exposure of a beam of laser light is approximately equal to the grid voltage V.sub.G, the surface electric potential Vo on the photoconductive drum 41 can be controlled by changing the grid voltage V.sub.G. Further, the surface electric potential Vo immediately after electrically charging it and prior to the exposure of a beam of laser light is detected by a Vo sensor 44 of a surface electrometer. It is to be noted that the surface electric potential on the photoconductive drum 41 becomes the above-mentioned surface electric potential Vo, when an exposure level EXL of a beam of laser light (referred to as an exposure level hereinafter) is a minimum value thereof (zero in the present preferred embodiment) even after the exposure of a beam of laser light.
In the first place, prior to the exposure of a beam of laser light, a negative surface electric potential Vo is set on the photoconductive drum 41 by the corona charger 43 thereby supplying an amount of electric charge corresponding to the surface electric potential Vo thereto, and then, a negative developing bias voltage V.sub.B (.vertline.Vo.vertline.&gt;.vertline.V.sub.B .vertline.) of a relatively low electric potential is applied to the developing roller 45r by a developing bias voltage V.sub.B generator 244. In this case, the surface electric potential of a developing sleeve of the developing device 45r is also set to the developing bias voltage V.sub.B.
Upon the exposure of a beam of light, an electric potential at an exposed position on the photoconductive drum 41 is lowered so as to change from the surface electric potential Vo to an attenuated electric potential of an electrostatic latent image or a surface electric potential V.sub.I after the exposure of a beam of laser light. The surface electric potential V.sub.I upon a maximum exposure level EXL is referred to as a surface electric potential V.sub.Im hereinafter.
When the attenuated surface electric potential V.sub.I becomes lower than the developing bias voltage V.sub.B, the toner transported onto the surface of the developing sleeve of the developing device 45r adheres onto the surface of the photoconductive drum 41. In this case, it is necessary to fall a difference between the surface electric potential Vo and the developing bias voltage V.sub.B into a predetermined range, and also the adhering toner amount becomes larger as a developing voltage .DELTA.V=.vertline.V.sub.B -V.sub.I .vertline. becomes higher. On the other hand, the attenuated surface electric potential V.sub.I changes depending on the surface electric potential Vo even upon the same exposure level. Accordingly, for example, when the surface electric potential Vo and the developing bias voltage V.sub.B are changed making a difference between the surface electric potential Vo and the developing bias voltage V.sub.B constant, a difference between the developing bias voltage V.sub.B and the surface electric potential V.sub.I changes, and then, the adhering toner amount changes, thereby controlling an image reproduction density of a reproduced image.
According to the image density control process of this type as described above, the maximum image reproduction density is made constant by automatically or manually by an operator's changing the surface electric potential Vo on the photoconductive drum 41 and/or the developing bias voltage V.sub.B.
In the automatic image density control process, first of all, a reference toner image of a reference image pattern which becomes a reference for the image density control process is formed on the surface of the photoconductive drum 41, and a light amount of a reflected light from the reference toner image is detected by an automatic image density control sensor (referred to as an AIDC sensor hereinafter) 210 provided in the vicinity of the photoconductive drum 41, thereby measuring an image reproduction density of the reference toner image. Data of the detection value detected by the AIDC sensor 203 are inputted to a printer controller 201, which in turn controls the grid voltage V.sub.G generator 243 and the developing bias voltage V.sub.B generator 244 in accordance with a comparison result between the data of the detection value detected by the AIDC sensor 203 and a predetermined value. The above-mentioned process is repeated until the adhering toner amount becomes the predetermined value.
In this case, in order to prevent any fog from being formed on a background of an image and to prevent carrier included in a developer including two components from adhering onto the photoconductor of the photoconductive drum 41, the image density control process is performed making a difference between the surface electric potential Vo and the developing bias voltage V.sub.B constant, conventionally.
As described above, however, upon performing the image density control process by changing the surface electric potential Vo and the developing bias voltage V.sub.B making the difference between the surface electric potential Vo and the developing bias voltage V.sub.B constant, when the grid voltage V.sub.G for inducing the above-mentioned surface electric potential Vo and the developing bias voltage V.sub.B are set to minimum values of adjustable ranges of the output voltages of the V.sub.G generator 243 and the V.sub.B generator 244, there is caused such a problem (referred to as a first problem hereinafter) that there may be reproduced an image having a maximum image reproduction density higher than a desirable maximum image reproduction density due to change in the photoconductor characteristic of the photoconductive drum 41, change in the circumstances or the like. On the other hand, when the grid voltage V.sub.G and the developing bias voltage V.sub.B are set to maximum values of the adjustable ranges thereof, there is caused such another problem (referred to as a second problem hereinafter) that there may be reproduced an image having a maximum image reproduction density lower than a desirable maximum image reproduction density due to change in the photoconductor characteristic of the photoconductive drum 41, change in the circumstances or the like. Therefore, when the above-mentioned first or second problem is caused, there can not be obtained a desirable gradation characteristic, and there can not be always obtained a reproduced image having a constant gradation reproducibility for an original.
By the way, there is another point to be considered in the automatic image density control process for automatically an image reproduction density by changing the grid voltage V.sub.G and the developing bias voltage V.sub.B.
In a digital image forming apparatus, in particular, a digital full color image forming apparatus, it is one of important problems to be solved to remove a fog which may be formed on a background of a paper. Since the fog is formed fundamentally depending on the grid voltage V.sub.G and the developing bias voltage V.sub.B, it is necessary to properly control both of the voltages V.sub.G and V.sub.B in order to prevent any fog. Generally speaking, it is known to those skilled in the art that the fog may be formed due to, mainly, deterioration of the photoconductor of the photoconductive drum 41 caused after an endurance limit thereof. Therefore, an image reproduction density is adjusted by changing the grid voltage V.sub.G keeping the surface electric potential Vo so as to be set to that in the initial state, thereby preventing any fog. Further, it is necessary to perform the above-mentioned fog removal process so as not to influence the automatic image density control process.
Furthermore, when a half-tone image is formed, it is necessary to take into consideration an influence to a gradation correction process which is generally called a .gamma. correction process. Generally speaking, a relationship between an original density of a read original image to be reproduced and an image reproduction density of a reproduced image becomes a non-linear .gamma. characteristic. Therefore, it is necessary to previously perform the gradation correction process or the .gamma. correction process for correcting data of an light amount of a beam of laser light to the original density so as to heighten faithfulness of the reproduced image. However, when the grid voltage V.sub.G and the developing bias voltage V.sub.B are changed, the .gamma. characteristic changes at that time. Therefore, there is caused such a problem that the faithfulness of the reproduced image is lowered unless the .gamma. correction process is performed depending on the changed .gamma. characteristic.